"You will remember, Watson," he remarked one afternoon, "that
there is a single common point of resemblance in the varying
reports which have reached us. This concerns the effect of the
atmosphere of the room in each case upon those who had first
entered it. You will recollect that Mortimer Tregennis, in
describing the episode of his last visit to his brother's house,
remarked that the doctor on entering the room fell into a chair?
You had forgotten? Well I can answer for it that it was so.
Now, you will remember also that Mrs. Porter, the housekeeper,
told us that she herself fainted upon entering the room and had
afterwards opened the window. In the second case--that of
Mortimer Tregennis himself--you cannot have forgotten the
horrible stuffiness of the room when we arrived, though the
servant had thrown open the window. That servant, I found upon
inquiry, was so ill that she had gone to her bed. You will
admit, Watson, that these facts are very suggestive. In each
case there is evidence of a poisonous atmosphere. In each case,
also, there is combustion going on in the room--in the one case a
fire, in the other a lamp. The fire was needed, but the lamp was
lit--as a comparison of the oil consumed will show--long after it
was broad daylight. Why? Surely because there is some
connection between three things--the burning, the stuffy
atmosphere, and, finally, the madness or death of those
unfortunate people. That is clear, is it not?"

"It would appear so."

"At least we may accept it as a working hypothesis. We will
suppose, then, that something was burned in each case which
produced an atmosphere causing strange toxic effects. Very good.
In the first instance--that of the Tregennis family--this
substance was placed in the fire. Now the window was shut, but
the fire would naturally carry fumes to some extent up the
chimney. Hence one would expect the effects of the poison to be
less than in the second case, where there was less escape for the
vapour. The result seems to indicate that it was so, since in
the first case only the woman, who had presumably the more
sensitive organism, was killed, the others exhibiting that
temporary or permanent lunacy which is evidently the first effect
of the drug. In the second case the result was complete. The
facts, therefore, seem to bear out the theory of a poison which
worked by combustion.

"With this train of reasoning in my head I naturally looked about
in Mortimer Tregennis's room to find some remains of this
substance. The obvious place to look was the talc shelf or
smoke-guard of the lamp. There, sure enough, I perceived a number
of flaky ashes, and round the edges a fringe of brownish powder,
which had not yet been consumed. Half of this I took, as you
saw, and I placed it in an envelope."

"Why half, Holmes?"

"It is not for me, my dear Watson, to stand in the way of the
official police force. I leave them all the evidence which I
found. The poison still remained upon the talc had they the wit
to find it. Now, Watson, we will light our lamp; we will,
however, take the precaution to open our window to avoid the
premature decease of two deserving members of society, and you
will seat yourself near that open window in an armchair unless,
like a sensible man, you determine to have nothing to do with the
affair. Oh, you will see it out, will you? I thought I knew my
Watson. This chair I will place opposite yours, so that we may
be the same distance from the poison and face to face. The door
we will leave ajar. Each is now in a position to watch the other
and to bring the experiment to an end should the symptoms seem
alarming. Is that all clear? Well, then, I take our powder--or
what remains of it--from the envelope, and I lay it above the
burning lamp. So! Now, Watson, let us sit down and await
developments."